1. Field of the Invention
The invention relates to controls for inductive devices and, more particularly, to a discharge device for a large inductive device such as an electromagnet.
2. Description of the Related Art
Large inductive devices are oftentimes incorporated into industrial applications. For example, an electromagnet is merely an encased inductor. Electromagnets can be used with a crane as lifting magnets in the steel industry for scrap material handling. The electromagnet is coupled to a magnet controller, which includes an electrical circuit that typically receives a DC voltage from a generator or other source and controls the voltage applied to, and thus the current flow through, the electromagnet.
The industrial applications in which these inductive devices are incorporated are such that the device and any controller are subject to harsh conditions in which damage to the equipment can easily result. For example, the conductors providing the power supply to a lifting magnet can be cut or otherwise disconnected from the lifting magnet. If the device is energized when it is abruptly disconnected from its power supply, a voltage level across its terminals can result high enough to damage or destroy the device. The typical rating of insulation for a lifting magnet is 600 volts. If the conductors are cut or otherwise disconnected, the voltage potential across the terminals of the lifting magnet can go as high as 10,000 volts, high enough to breakdown the insulation. The breakdown of the insulation can destroy the electromagnet, which is expensive to repair and/or replace.
In the case of electromagnets, one solution proposed has been the addition of a spark gap surge arrester including two electrodes, each connected to a separate terminal of the electromagnet. When the voltage level is high enough, the air gap between them breaks down. The resultant spark discharges the energy from the electromagnet. However, such arresters are exposed to damage themselves due to the harsh environments in which they are located.